An oxide film can be deposited on a substrate surface using a known method generally referred to as plasma enhanced chemical vapor deposition ("PECVD"), also known as glow discharge deposition. This known method is used commercially, for example, to deposit films of photovoltaic material on a substrate in the production of solar cells. PECVD involves ionizing a gas mixture which includes the species to be deposited. For a film of tungsten oxide to serve as an electrode in an electrochromic device, for example, the gas mixture, might comprise tungsten hexafluoride, oxygen and hydrogen gases.
Deposition of an oxide by PECVD involves a dynamic two-way process in which deposition and its opposite, surface etching, are occurring simultaneously. Conditions must be controlled to favor deposition over etching. Specifically, parameters such as the composition of the deposition atmosphere, the deposition temperature and ion bombardment energy, that is, the strength of any electrical field applied to accelerate ions of the plasma toward the substrate surface to be coated, as well as secondary parameters such as flow rate and pressure, and power level, all must be controlled to achieve deposition. More precisely, these factors must be controlled to achieve net deposition, that is, to sufficiently favor deposition over etching in the dynamic process that the net result is an accumulating deposit of the desired oxide.
Significantly, higher quality films, in particular, higher electronic quality films, are known to be achieved at PECVD deposition conditions only marginally favoring deposition over etching. Thus, for example, to achieve high quality films of crystalline tungsten oxide, the deposition conditions preferably are maintained close to those favoring etching. Unfortunately, however, oxide films often are not readily formed on certain smooth surfaces at such marginal PECVD conditions. There is some speculation that this deposition difficulty may be due to a lack of nucleation sites on such smooth surfaces. In any event, there has been difficulty in using PECVD techniques to form high quality oxide films on certain substrates, such as glass and electronic quality dielectric substrates. While directed to deposition of a metal, tungsten, rather than an oxide, the difficulty of chemical vapor deposition onto dielectric surfaces, such as silicon dioxide, is shown generally in U.S. Pat. No. 4,777,061 to Wu et al. The Wu et al patent suggests an argon plasma pretreatment of the dielectric surface followed by low power plasma deposition of tungsten, followed by thermal deposition of tungsten. That is, the Wu et al patent teaches that plasma deposition is terminated after deposition of an adhesion film, following which deposition proceeds by thermal decomposition. Plasma enhanced chemical vapor deposition also is discussed in U.S. Pat. No. 4,572,841 to Kaganowicz et al. An improved film having increased density is said to be achieved in the Kaganowicz patent by introducing excess hydrogen into the gaseous atmosphere during deposition. An optional first step is said to involve oxidizing the surface of the silicon substrate by a plasma oxidation method.
It is an object of the present invention to provide a PECVD deposition method to form oxide films on substrate surfaces. This and additional objects and features of the invention will be better understood from the disclosure and discussion which follow.